Preweakened on chip metal fuse using dielectric trenches for barrier layer isolation

ABSTRACT

A fuse for use in an integrated circuit includes a dielectric layer into which a trench or void is etched defined by a top opening and a bottom floor. The trench includes at least one undercut which forms an overhang in the dielectric layer partially shielding the bottom floor. A second or barrier layer deposited onto the dielectric layer is interrupted or non-continuous at the undercut. A third, or electrically conductive layer, is electrically continuous over the fuse. A weak spot in the third layer exists in the lack of structural support by the second layer at the interruption. A further weak spot in the third layer exists in the electrical isolation of the conductor layer, i.e. no leakage current through the barrier layer, at the interruption.

BACKGROUND OF THE INVENTION

The invention relates to the art of semiconductor manufacture. It findsspecific application for fuses within an integrated circuit and will bedescribed with particular reference thereto. However, it is to beappreciated that the present invention is also amenable to PROM-typedevices, trimming resistors and other like devices.

Fuses have long been important circuit elements. While fuses aregenerally used for circuit protection and fire prevention, in integratedcircuits they are also used as a means of calibration, programming andproviding data security. For example, fuses can be used for trimming orcalibrating certain parameters in a circuit, or for recordinginformation by selectively blowing fuse elements in an array, or forpreventing information stored in a memory device from being accessed.Whether the fuse exists for circuit protection or to achieve certainoperating parameters the fuses are designed to change state undercertain conditions. Indeed, one way to change state is a to blow a fuse.This is accomplished by applying a sufficiently high current through thefuse. The heating that results causes the conductor to melt or vaporize.In this way, the fuse is transformed from its conductive state to ablown, or open circuit state.

The conductors used to fabricate fuses on integrated circuits are madeof layers of conductive metal, typically aluminum or aluminum compound.For reasons unrelated to fusing, the aluminum layer is deposited over abarrier layer typically comprising titanium and/or a titanium compound.Unfortunately, the addition of titanium has an adverse effect on fusereliability. The adverse effect is caused by the high melting point oftitanium as compared to the aluminum bulk conductor layer. Thus, whencurrent levels are applied to the metal stack which should effect ablown fuse, sometimes portions of the barrier layer remain. Thisremaining portion of the barrier provides a conductive path around the‘blown’ fuse. In other words, current levels that do melt the bulkconductor sometimes do not completely melt the barrier layer. Thispartial path undesirably allows a current path, defeating the intendedopen circuit.

Accordingly, it is desirable to provide a technique for manufacturing amore reliable fuse, using existing methods of applying the metal stack.

The present invention contemplates a new and improved method andapparatus fuse overcomes the above-referenced problems and others.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, a circuitelement which interrupts electrical conductivity in a portion of acircuit includes a first layer providing adhesion for the circuitelement. A trench is also provided in the first layer and a secondlayer, overlaying the first layer, has an interruption in the trench. Athird layer including electrically conductive material, overlays thesecond layer.

In accordance with another aspect of the present invention, the trenchdefines a volume in the first layer bounded by an opening in the firstlayer, an undercut area in communication with the opening, and a bottomfloor. The bottom floor is larger than the opening in the first layer.

In accordance with another aspect of the present invention, the firstlayer includes an overhang which shields the portion of the bottom floorof the trench. In accordance with another aspect of the presentinvention, the circuit element further includes a weak spot in the thirdlayer adjacent to the interruption.

In accordance with another aspect of the present invention, the weakspot includes an area of reduced mechanical strength resulting from alack of structural support by the second layer in the area of theinterruption.

In accordance with another aspect of the present invention, the weakspot includes an area of reduced mechanical strength in the third layerresulting from crystalographic changes within the third layer in thetrench.

In accordance with another aspect of the present invention, the secondlayer includes conductive material, and the weak spot includes an areawhere electrical communication is only provided through the third layer.

In accordance with another embodiment of the present invention, in anintegrated circuit fuse including an electrical conductor having areduced cross sectional area in a first preweakened area, the fuseincludes: an undercut trench on top of a dielectric layer, and a secondpreweakened area in the electrical conductor adjacent to the trench.

In accordance with another aspect of the present invention, the fusefurther includes a barrier layer applied to the dielectric layer with atleast one gap. The electrical conductor is applied over the barrierlayer and over the gap so as to create a fuse with a second preweakenedarea being adjacent to the gap.

In accordance with another aspect of the present invention, the barrierlayer includes a layer of titanium or a titanium compound and theelectrical conductor includes a layer of aluminum or an aluminumcompound.

In accordance with another aspect of the present invention, the fusefurther includes a barrier layer disposed between the dielectric layerand the metal layer, where the barrier layer has a discontinuity in thevoid.

In accordance with another aspect of the present invention, the weakpoint includes a mechanical weakness from the discontinuity in thebarrier layer.

In accordance with another aspect of the present invention, the weakpoint includes an electrical weakness from the discontinuity of thebarrier layer in the neck down region of the metal layer adjacent to thevoid.

In accordance with another aspect of the present invention, the voidincludes an undercut which shields a portion of the bottom floor by anoverhanging edge of the top opening during application of the barrierlayer. The overhanging edge establishes the discontinuity in the barrierlayer.

In accordance with another aspect of the present invention, the undercutincludes an acute angle between a side wall and the bottom floor of thedielectric.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention in may take form in various components and arrangements ofcomponents, and in various steps and arrangements of steps. The drawingsare only for purposes of illustrating preferred embodiments and are notto be construed as limiting the invention.

FIG. 1 is a sectional view of a dielectric layer suitable to practicethe present invention;

FIG. 2 is a sectional view of the dielectric layer of FIG. 1 with abarrier layer applied;

FIG. 3 is a sectional view of the dielectric layer and barrier layer ofFIG. 2 with additional conductor layers applied to form a fuse suitableto practice the present invention;

FIG. 4 is a plan view of the fuse of FIG. 3; and,

FIG. 5 is a plan view of an alternate embodiment of a fuse sufficient topractice the present invention.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIG. 1, a Interlevel dielectric (ILD) 10 is shown witha trench 12 formed or etched thereinto. The trench 12 defines a volumein the ILD 10 bounded by a bottom floor 14, a top opening 16 and anundercut area 20. The undercut area 20 effectively results in aprotrusion or overhang 22 in the ILD 10 which partially shields orcovers the bottom floor 14. Those skilled in the art will appreciatethat while the illustrated and preferred overhang 22 comes to a sharpwell defined point, any overhang which partially shields or covers thebottom floor 14 is also effective. Preferably, the trench 12 andundercut 20 are formed together during isotropic plasma etching or wetchemical etching. However, formation of the trench 12 and/or overhang 22by etching is not limiting. Aside from employing other mechanisms toremove dielectric, voids within the dielectric can be achieved bygrowing substrate protrusions, layering substrates having variably sizedtop openings, and the like.

Referring now to FIG. 2, a barrier layer 30 is applied over the ILD 10.Preferably barrier layer 30 comprises titanium and/or a titaniumcompound as is typical in the art. Moreover, those skilled in the artwill appreciate that the barrier layer typically is provided to reduceadverse affects and increase the performance and reliability ofintegrated circuits. Exemplary benefits of titanium based barrier layersinclude reduced electromigration and/or hillock formation. Metal layers,such as the barrier layer 30, are typically applied from the top ordistal side of the ILD 10 by sputtering or other application processes.As the barrier layer 30 is applied, the portion of the bottom floor 14shielded or covered by the overhang 22 does not accumulate as much ofthe barrier layer 30 material as other, non-shielded areas. For example,and with continued reference to FIG. 2, present barrier layers oftitanium and/or titanium compound are typically applied 1000 Å or lessin thickness. This thickness of the barrier layer 30 partially definesdimensions of the trench 12. Continuing with the above example,preferably trench 12 has a length L and a somewhat smaller depth D. Inorder for the depth D to accommodate the barrier layer 30, the depth Dmust be greater than 1000 Å. Preferably, the depth D is between 2 and 5times the barrier layer 30 thickness. The ratio of depth to length isreferred to as the aspect ratio. In the present preferred embodiment,D/L will range from 0.2-0.5. In other words, the length of trench 12 ispreferably 2-5 times the depth D.

Continued reference to FIG. 2 reveals that the combination undercut 20and overhang 22 interrupt the continuity of barrier layer 30 resultingin a gap or interruption 34. This interruption 34 serves to act as anelectrical weak spot in the fuse because no alternate current pathexists through the barrier layer 30, as will be more fully discussedbelow.

Referring to FIG. 3, a bulk conductor layer 40 is applied over thebarrier layer 30. In the preferred embodiment, the bulk conductor 40 ismade of a layer of aluminum or aluminum compound. The geometry of theILD 10 and barrier layer 30 impart a weak spot 44 in the bulk conductor40 as it is applied. A mechanical weakness is also created at the weakspot 44 by the interruption 34 behind the bulk conductor 40. Moreover,crystallographic changes in certain conductors, such as aluminum oraluminum compound, occur as the bulk conductor 40 material fills thetrench 12 further mechanically weakening the weak spot 44. As notedabove, those skilled in the art will appreciate that in addition tomechanical weakness 44, an electrical weak spot exists. The electricalweakness in weak spot 44 exists because the gap or interruption 34 inthe barrier layer 30 removes any alternate conductive path (i.e. throughthe barrier layer 30) desirably forcing all current through theconductive layer 40. Accordingly, higher current densities in this areamake the fuse likely to change state or blow in this area.

Referring now to FIG. 4, a plan form view shows the fuse 60 having alarge metal area 62, a neckdown fuse region 64 with a smaller metal areahaving a width W, and the generally square trench 12 having sides oflength L. Those skilled in the art will recognize that the square shapeof the trench is more a function of semiconductor masking and etching,and accordingly, other shaped trenches could be employed with equalefficacy. However, the preferred embodiment of the present inventionutilizes a length L at least twice the width W of the neckdown region ofthe fuse 60. Additionally, those skilled in the art will appreciate thatmultiple weak spots 44 can be placed into the fuse space by (1)establishing undercuts 20, 20′ (FIG. 3) on each side of the trench 12,and/or by (2) placing multiple trenches (shown in ghost) in series alongthe fuse path as space allows.

FIG. 5 is an alternate embodiment of a fuse suitable to practice thepresent invention. Similar to FIG. 4, the neckdown fuse region 64extends across trench 12′. However, unlike the FIG. 4, embodiment theneckdown region 64 only forms one weak spot 66. The advantage of FIG. 5lies in its potential for increased density of fusible lengths. In otherwords, the arrangement uses less physical space allowing for smallerIC's or other features on existing IC's. Those skilled in the art willrecognize that more weak spots are possible in the embodiment of FIG. 5by including additional trenches in either the neckdown region alone oron an opposite side partially underlying large metal area 62.

The invention has been described with reference to the preferredembodiments. Obviously, modifications and alterations will occur toothers upon reading and understanding the preceding detaileddescription. It is intended that the invention be construed as includingall such modifications and alterations insofar as they come with thescope of the appended claims or the equivalents and thereof.

Having thus described the preferred embodiments, the invention is nowclaimed to be:
 1. A method for the manufacture of a semi-conductor fusecomprising: forming a trench into a dielectric material, the trenchdefining a top opening in the dielectric material, a bottom floor largerthan the top opening, and side walls therebetween; and, applying anelectrically conductive metal layer across the dielectric material andtrench, said metal layer having a weak point between the top opening andthe bottom floor.
 2. The method as set forth in claim 1, furthercomprising before the applying step: depositing a barrier layer over thetrench such that the barrier layer is interrupted between the topopening and the bottom floor.
 3. The method a set forth in claim 2,wherein the weak point is an electrical weak point due to theinterruption in the barrier layer.